With the rapid advancement of various communication systems with the use of electromagnetic waves in the microwave area including mobile communication such as cellular phones and satellite broadcasting, a great number of dielectric materials have been developed. In these dielectric materials, priorities are given on three dielectric characteristics, namely, dielectric constant (∈r), unloaded quality factor (Qu) and temperature factor (τf) of resonance frequency. In general, such a dielectric material should have a high ∈r, a large Qu and a small absolute value of τf. However, it is highly difficult to meet all of these requirements at the same time, since they are incompatible with each other. Therefore, it is desired to control each of these characteristics within a range according to need.
Known dielectric materials to be employed for the above purposes include BaO—ZnO—Ta2O5 based materials (BaZnTa-based materials) having ∈r of about 20 to 30 which are disclosed in the following JP-B-59-48484, and BaO—RE2O3—TiO2 (RE: rare earth element) based materials (BaRETi-based materials) having ∈r of about 60 to 80 which are disclosed in the following JP-B-59-37526. These materials have been employed in practice as materials for high-frequency resonators and filters. However, few materials can still exert intermediate ∈r values and thus it has been required in recent years to develop materials having intermediate ∈r values and being available for high-frequency purposes.
As such materials, CaO—TiO2—Al2O3—Nb2O5 based materials (CaTiAlNb-based materials) disclosed in JP-A-2001-302331 and JP-A-2001-302333, and JP-A-2002-308670 filed by the present inventors are known.